This invention relates to a construction of a negative chamber for a storage battery of the sodium-sulfur type, and is especially intended to simplify manufacturing of storage batteries and provide improved, utilization of sodium, more uniform electric current distribution on the solid electrolyte tube surface, and safety.
The sodium-sulfur storage battery is a high-temperature type secondary battery in which sodium as a negative reactant and sulfur as a positive reactant are separated by a .beta."-alumina as a sodium-ion conductive solid electrolyte tube, and are actuated at a temperature of 300.degree.-350.degree. C. A vertical sectional view of a conventional sodium-sulfur storage battery is shown in FIG. 1, and operating conditions and problems to be considered will be described hereunder according to FIG. 1. 1 is a .beta."-alumina as a solid electrolyte tube. 2 is an .alpha.-alumina ring to which an upper open part of the solid electrolyte tube 1 is jointed with solder glass. 3 is stainless steel fiber. 4 is sodium as a negative reactant impregnated in the stainlss fiber 3. 5 is a negative current collector terminal which vacuously impregnates the negative reactant and functions also as a current collector. 6 is a negative cover comprising stainless steel, and the negative current collector terminal is welded to a central part thereof. 7 is a negative auxiliary cover comprising such as stainless steel, iron, Fe-Cr-Al alloy etc. 8, 9 are aluminum rings. 10 is a battery housing comprising metal which has sufficient resistance to chemical and electrochemical attack by molten sulfur and sodium polysulfide such as, for example: stainless steel, Fe-25Cr-4Al, Fe-Cr-Al-Y alloy, chrome diffusion processed stainless steel etc; and functioning also as a positive current collector. The negative auxiliary cover 7 and the battery housing 10 are thermocompressively jointed respectively through the aluminum ring 8 and the aluminum ring 9 to an upper and lower faces of the .alpha.-alumina ring 2 to which the solid electrolyte tube 1 is jointed with solder glass. After completion of the thermocompression jointing work, the stainless steel fiber 3 is filled in the solid electrolyte tube 1, the negative cover 6 welded with the negative current collector terminal 5 is inserted therein, then the negative cover 6 and the negative auxiliary cover 7 are welded together. After the above-mentioned component body is heated up to about 150.degree. C., molten sodium of constant quantity is vacuously impregnated through the negative current collector terminal, then the negative current collector terminal is vacuously sealed. 11 is sulfur as a positive reactant. 12 is a positive electroconductive material, which comprises fiber such as graphite, corbon etc., impregnated with the positive reactant 11, and inserted from the bottom of the battery housing 10. A bottom cover 13 is fitted to the battery housing and welded thereto under inert gas atmosphere or vacuum. A primary problem to be considered at a manufacturing stage of the above-mentioned conventional sodium-sulfur storage battery is unevenness of filling density of the stainless fiber to be filled in the solid electrolyte tube 1 and hardness of filling work thereof. A secondary problem is heat cycle applied on the solid electrolyte tube 1 due to necessary heating of the component body including the solid electrolyte tube 1 up to above the melting point of sodium, and influence on productivity caused by times required for rising and lowering temperature, when the negative reactant is filled. Now problems, which will arise when a storage battery manufactured as shown in FIG. 1 is heated and galvanized, will be discussed here under.
The first problem is uneven wetting of sodium on the inside surface of the solid electrolyte tube 1 accompanied by a decrease of sodium in the stainless steel fiber 3 when discharging. This phenomenon is attributable to a suction effect due to the fact that capillarity of the stainless steel fiber lessens with a decrease in a quantity of sodium to produce unwetted portions which may cause breakage of the solid electrolyte tube 1 due to uneven current density generated therein. Secondarily, when the solid electrolyte tube 1 is broken due to the above-mentioned reason, or when it is broken by being applied with mechanical shock etc., sodium reacts directly with sulfur or sodium polysulfide 11 to generate a large amount of heat and melt the battery housing 10, thus even an adjacent storage battery would be broken. The third one is a reduction in the utilization factor of sodium due to a decrease in discharge capacity caused by the above-mentioned first problem, and lessening of the quantity of sodium available for reaction as compared with the quantity of sodium remaining in the solid electrolyte tube 1.
An object of this invention is to overcome the above-mentioned problems, i.e. to simplify the manufacturing of a storage battery, to improve the utilization factor of sodium, to improve service life of the battery by uniformalizing distribution of current on the surface of the solid electrolyte tube, and to improve safety by preventing the battery housing from being broken even in case of breakage of the battery itself.